Method not found: GetNamedPermissionSet() error on installing .net

Hi,

After upgrading to .net 3.5, my web app is giving the error on
all .aspx pages:

" Method not found: 'System.Security.NamedPermissionSet
System.Web.HttpRuntime.GetNamedPermissionSet()' "

....on an app which worked just fine before.

This is on XP SP2, IIS 5.1. Have tried aspnet_regiis -i, reinstalling
IIS, and reinstalling the .net framework.

Stack trace and event log entry below.

At a loss what to do next - don't want to reinstall everything as this
is my development workstation. Any further suggestions welcome.

Cheers,
A.
--
Event code: 3005
Event message: An unhandled exception has occurred.
Event time: 03/01/2008 13:53:49
Event time (UTC): 03/01/2008 13:53:49
Event ID: df4c8c8fb96348fc8f79fa0c416c510c
Event sequence: 9
Event occurrence: 6
Event detail code: 0

Application information:
Application domain: /LM/W3SVC/1/Root/
***myapp***/-1-128438410780466912
Trust level: Full
Application Virtual Path: /tinyvan
Application Path: C:\Projects\***myapp***\
Machine name: ***mycomputer***

Process information:
Process ID: 604
Process name: aspnet_wp.exe
Account name: ***mycomputer***\ASPNET

Exception information:
Exception type: MissingMethodException
Exception message: Method not found:
'System.Security.NamedPermissionSet
System.Web.HttpRuntime.GetNamedPermissionSet()'.

Request information:
Request URL: http://localhost/***myapp***/login.aspx
Request path: /***myapp***/login.aspx
User host address: 127.0.0.1
User:
Is authenticated: False
Authentication Type:
Thread account name: ***mycomputer***\ASPNET

Thread information:
Thread ID: 1
Thread account name: ***mycomputer***\ASPNET
Is impersonating: False
Stack trace: at
System.ServiceModel.PartialTrustHelpers.get_NeedPartialTrust Invoke()
at
System.ServiceModel.PartialTrustHelpers.PartialTrustInvoke(C ontextCallback
callback, Object state)
at System.ServiceModel.Activation.HttpModule.ProcessRequest(Obj ect
sender, EventArgs e)
at
System.Web.HttpApplication.SyncEventExecutionStep.System.Web .HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step,
Boolean& completedSynchronously)